U–Pb and Al–Mg systematics of the ungrouped achondrite Northwest Africa 7325
U–Pb and Al–Mg systematics of the ungrouped achondrite Northwest Africa 7325
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DOI：
10.1016/j.gca.2016.03.028
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发表时间：
2016-06
影响因子：
5
通讯作者：
P. Koefoed;Y. Amelin;Q. Yin;J. Wimpenny;M. Sanborn;T. Iizuka;A. Irving
中科院分区：
文献类型：
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作者：
P. Koefoed;Y. Amelin;Q. Yin;J. Wimpenny;M. Sanborn;T. Iizuka;A. Irving
西北非洲（NWA）7325是一个独特的未分组辉长岩无球粒陨石，其特征与水星的可能联系一致。为了了解该陨石的成因和母体性质，我们用长寿命的U-Pb和短寿命的Al-Mg计时器测定了它的结晶年龄。假设NWA 7325的238 U/235 U比值与地球和太阳系的整体值137.794相同，则由六个酸浸辉石组分确定的内部Pb-Pb等时线得出的年龄为4563.4 ± 2.6 Ma。7个组分（4个斜长石、1个辉石、1个橄榄石和1个全岩）的Al-Mg同位素分析确定了对应于26 Al/27 Al 0 =（3.03 ± 0.14）× 10− 7的回归线和0.093 ± 0.004‰的初始δ 26 Mg δ。当锚定在D 'Orbigny角闪石上时，初始26 Al/27 Al的年龄为4563.09 ± 0.26Ma。4563.4 ± 2.6Ma的Pb-Pb年龄与4563.09 ± 0.26Ma的Al-Mg年龄完全一致，但NWA 7325的低U浓度导致Pb-Pb年龄精度较低。初始δ 26 Mg同位素的过剩可以用27 Al/24 Mg分馏和行星分化后Mg同位素演化来解释。此外，NWA 7325的母岩浆最有可能在富钙铝包裹体（CAI）形成后的1.72 Ma内形成。NWA 7325与淬冷的角闪石和一些未成组的无球粒陨石几乎同时形成，形成时间约为4563 Ma，这表明在CAI形成后的4500万年至500万年间，有大量的行星体形成并分化。这个古老的年龄可能被解释为反对NWA 7325起源于水星的论点，但它并没有完全排除它。
Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U–Pb and short-lived Al–Mg chronometers. An internal Pb–Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4 ± 2.6 Ma, assuming that the238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al–Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to26Al/27Al0= (3.03 ± 0.14) × 10−7and an initial δ26Mg∗of 0.093 ± 0.004‰. When anchored to the D’Orbigny angrite, this initial26Al/27Al yields an age of 4563.09 ± 0.26 Ma. The Pb–Pb age of 4563.4 ± 2.6 Ma and Al–Mg age of 4563.09 ± 0.26 Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb–Pb age. The observed excess in initial δ26Mg∗can be explained by27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72 Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563 Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4–5 Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.